…about ruminants experiencing heat stress

This year the summer (and heat stress) arrived suddenly. It’s therefore important to familiarize yourself with the possible effects of heat stress on ruminants.

Here are 12 interesting facts about ruminants experiencing heat stress:

1. Both high temperature and high humidity can contribute to heat stress in animals and is expressed as THI (Thermal Heat Index).
   

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2. The thermal comfort range of lactating dairy cows was estimated to be from -0.5 to 25°C (Berman et al. 1985).
   

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3. According to the re-evaluation of Burgos Zimbelman and Collier (2011), it appears that a THI of 68 is low enough to have a negative effect on production.
   

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4. There is normally a lag time between heat stress and the effect it has on production  (West et al. 2003).
   

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5. Ruminants decrease feed intake to create less metabolic heat, as the heat increment of feeding is a large portion of whole body heat production (Kadzere et al. 2002).
   

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6. Heat stressed cows could experience up to 30% increase in maintenance cost (Fox and Tylutki, 1998).
   

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7. Drop in DMI explains only 35 to 50% of milk loss (Baumgard & Rhoads, 2012).
   

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8. Cows experiencing heat stress, can go into a negative energy balance and would prefer energy from glucose rather than body fat reserves (NEFA) (Febbraio, 2001). This could be due to ß-oxidation of NEFA that may produce more metabolic heat than carbohydrates (Baumgard and Rhoads, 2007).
   

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9. Heat stress can decrease total rumen VFA content (Schneider et al. 1988), specifically propionate levels (Kelley at al., 1966). This finding is supported by Rhoads et al, 2009, who reported a decline in glucose concentrations during heat stress.
   

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10. Cows experiencing heat stress could display an increase in plasma urea nitrogen (PUN) (Wheelock et al., 2010). This could be due to inefficient rumen ammonia incorporation into microbial protein, or breakdown of skeletal muscle.
    

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11. Heat stress could have the largest effect on conception rate from day 42 before AI until day 40 post AI (Jordan, 2003).
    

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12. Heat stress can decrease size of foetus, especially during late gestation (Alexander et al., 1971) and can reduce oxygen consumption of the foetus up to 50% (Bell et al., 1987).
    

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